CA2073182C - Apparatus and method for recovering a time-varying signal in a serial data system - Google Patents

Abstract

Apparatus and method for recovering a time varying TDM signal packet (Rx) having a long duration. The packet (Rx) is recovered in a forward or reverse direction depending upon the intensity of the signal over the recovered portion of the packet (Rx). Known sync words (601) and codes (607) within the signal packet (Rx) provide starting points from which information in the packet (Rx) is recovered.

Description

Apparatus and Method for Recovaring a Tim~-Varying Si~nal in a Serial :)ata System Field of the Invention The present invention relatss generally to information signal recovery, and, rnore particularly, to an apparatus and method for recovsring information in a 10 time-varying signal which may have ogherwise boen lost in noise.

Background of the Invention 1~
The rapid expansion of the number of cellular radio telephones coupled with the desire to provide additional services has prompted the development of a new standard.
The standard suggests an increasa in system capaci~y 20 ovsr the current analog system through the use of digital modula~ion and speech coding teohniques. The standard uses a time division mulUplex (TDM) system which splits the current channel into six signal packets of which three are currently in use. A packet is a burst of information 26 characterized by sequentially enco~ed symbols for the intendad ~c~iver. The linear modulation techniqlle to transmit the digital information within tha channel is ~/4 DQPSK (differontial quadrature phase shifted keying).
The us~ qf 7~/4 DQPSK linear mo~ul~tion in the U.S.
30 Digi~al Ccllular system provides spec~ral efficiency allowing the use of 48.6 kbps channel data rates. ~e/4 DQPSK transmits the data information by enoodirlg consecutiv0 pairs of bits, commonly known as symbois, into one of four phase angles ( 7~14, +37c14) based upon .

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2 207~1~2 gray eneoding. These angles are then differen~ially encoded producing an 8 point constellation.
Cellular systems operate in the sxisting 800 MHz band. Radio pfop~g~lion at these frequencies is ~enerally 5 charactari~cl by three types of distortion: ~ime dispersion distortion, multipath distortion and lognormal distortion. Time dispersion distortion of a received signal occurs when a transmittcd si~nal is rec~ived via mors than one pr~p~lion path each having a differant 10 path leng~h. Measured received signals having ~ime dispersion distortion typically have a stron~ first component and multiple components that are generally lower in amplitude for larger delays. Time dispersion distor~ion of the received signal is usually found in an 15 environment wh~re a lar~e refleeting source, such as a mountain, is present. A mobile radio in this environment receives the signal from a fixed source transmittsr and the delayad signal from the reflacting sourco. The time delay b~tween the reception of the two signals results in 20 time disparsion distortion.
Multipath distortion is characterized by many rays of the same signai havin~ ~ilieren~ ener~y levsls reaching the receiver at the sams time. Tha number, phase and intensity of the signals received by the receiver in a 26 multipath environment may vary over time as a result of repositioning of the receivsr, or of the objects from which a ~ransmitted signal is reflected. As a result, the phase and signal level of a received signal varies over time. This variance is rsferred ~o as "fading" of the 30 signal. The resultan~ signal stren~th and rate of changs of signal sl-er,ylh at the receiver is predominantly determinsd by how rapidly the receiver is moving throu~h its environment, and the frequency of the channel being used. For ins2anee, in the eellular frequency band, and .

2~ 73~ ~2 when a cellular radio telephone is positioned in a vehiole traveling at 60 mph, the signal streng~h of the received signal can vary by approximately 20 decibels during a 5 millisecond period.
In th~ case of tima dispersion and multipath distortion, h~o received si~nals transmitted from ~he same source which are 180 d~grses out of phase effectively cancal each other out. The received signal's intensity approaches a null ànd tha rate of change of the received signal intensity over time is rapid. Since the raceived signal strength in~ensity is low, the modulated information can be corrupted by noise presen~ in the channel. A signal corrupted by noise can alter the state of the demodulated information thereby causing the recsiver to detect wrong information.
Lognormal distortion of a received signal occurs when the distance beh~een the transmitting source and the receiver increas~s ther~by causing a logarithmic decreasa in the signal strength at the reccivor. The distance at which lo~normal distortion begins depends upon the transmitter's signal powsr and the reeeiv0r's sensitivity. As the distance betwean the transmitting source and the receiv~ increas~s, the receiv~d signal strength intansity may dccrease to a lev~l whareby th~
moclul~te~ information is corrup~d by noise present in the channel. As with time dispersion and multipath distortion, a signal corrupted by noise can alter th0 state of the demodulated information th~reby causing th~
receiver to detec~ wrong information.
Racovering a si~nal packet having tims-varying signal intensity is feasible when the packet is relatively short. For sxample, variation of the signal intensity over a packet having a 0.5 millisecond duration is usually not significant enough to alter the state of the information in 2073~82 the packe~. If the entire packet is lost in noise, the performance of ~he system would not be substantially degraded. The packe~ with short duration contains less information than longer duration packets. The signal's 5 intensity is considered to be constant over the duration of the packet while the in~or,-,alion in the packet is recovered.
IlowevQr, systems which specify a signal packet havin~ a relatively long duration, for example, 6.66 10 millisecond duration in ~he U.S. Digital Celiùlar systsm, variation in the signal s~rsngth intensity can be si~nificant. Variations can cause the signal intensity to approach the noise floor of the channel thereby corrupting the information in the packet thereby causing the receiYer 15 to recover wrong information.
Thus, a formidable challenge is to provide a system for recovering information in a tirne-varying si~nal packet having a long duration.

2~731~2 Summary of the Invention Briefly stated, the inven~ion comprisas an apparatus for recovering a signal packet includad in a serial data 5 signal, wherein the packet is formed of multiple sequential symbols and a prede~ermined symbol. The predetarmined symbol has a predetermined posltion and value.
A receiver receives the serial data signal. A
10 direction of racovery is determined for the symbol sequential to the pred~termined symboi. The symbol is recovered in the determined direction. The recovery direction is changsd be~Neen the determined direction and a second direction. The rscovered symbol is stored.

2~73182 Brief Description of the Drawings FIG. 1 is a blook diagram of a TDM r0ceiver constructed in accordance with the present invention.
Fl(3. 2 is a biock diagram of a da~a recovery processor includ~d in the receiver in FIG. 1.
FIG. 3A is a block dia~ram of a phase detecter operating in a fon~ard procsssing mode included in ths d~ta r~overy processor of. FIG 2.
FIG. 3B is a block diagram o~ a phase cleteclor operating in a reverse processing mode included in the data recovery processor of FIG 2.
FIG. 4A is a block diagram of a loop filter operating in a forward processing mode included in the data recovery processor of FIG 2.
FIG. 4B is a blook di~ram of a loop filtsr opera~in~
in a reverse procsssing mode included in the data recovery p~cessor of FIG 2.
FIG. 5A is a block diagram of a differential decoder oparatin~ in a forward processing mode included in the da~a recovery processor of Fltà 2.
FIG. 5B is a block diayram of a differsntial deco~er operating in a reverse processing mode inciuded in the data racov~ry pr-,cessor of FIG 2. .
FIG. 6 repressnts a channel state dia~ram for a Tt: M
paeket sequence for a typical land-to-mobile station transmission which is utilized by the present invention.
FIG. 7 is a block diagram of an equalizer included in the data r~covery procassor of FIG. 2.

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2~73~82 Detailed Desoription of a Preferred Embodiment Referring now to the clrawings, a block dia~ram of a TDM receiver 100 shown in FIG. 1 has been construct~d in 5 accordance with the present invention. The TDM rc~eiver overcomes the aforementioned problems by recovering the signal packet in a forward and rsverse direction in time depanding on the in~ensity of the signal. Groups of known symbols called words, havirig fixed positions and values 10 within the packet, provide starting points from which the information is recovered. Tha packet is recovered in a direction responsive to the signal int~nsi$y over the duration of ths packet.
Ths receivsr 100 includes an antanna 101 which 15 couples the packet intended for ~he reoeiver 100 to a receive data buffer 103 and a mode controller 105. The receive data buffer 103 provides a storage location for the packet while its information is being recovered. A
control signal at line 109, genera~ecl by a mod(~ controller 20 105, is coupled to the receive data buffer 103 and a data recovery processor 107. Tha control signal at line 109 detarmines if the information in the signal packet will be couplsd from the receivcd data buffer 103 to the data recovery processor 107 in a forward or reversa direction.
25 The mode controller 105 generates the control signal at line 109 in rcsponse to the signal packe~ and a quality signal at line 111 coupled from the data recovery procassor 107.
The quality signai at lins 111 is a value related to 30 the quality of the signal's intensity varying with tirne over the duration of the packet. In alternate embodiments, the quality signal may also compris~ other signal parameters such as the signal's phase. If th0 intensity of the receiv~d signal approaches a null durin~
. ., 8 2~73182 information recovery, the mod~ controller 105 signals the receive data buffer 103 and the data recovery proc~ssor 107 to begin recovering the packe~ in a forward direotion ~rom one starting point un~il it reaches a null. The information in the paoke~ continues to be recovere~ from a n~w starting point on the other side of tha null in the reverse direc~ion until it reaches the null. Recovering the packet in both a forward and reverse direction in ~ime improves ths likelihood of detecting the correct information, thereby reducing the bi~ srror ra~e for the received signai.
In an alternate ernbodiment, the signal packet may be recovered in both a forward and reverse direction from mul~ipie predet3rmined starting points to multiple predetermined ending points~ For example, recovery of the packet may begin at a first starting point and recover data in the forward direction until it r~aches the ending point. Then rccovery continuas from a second s~arting point in a reverse clireclion until it reach~s ~he same ending point. Recovering data from multiple startin~
points improves the bit error rate for the received signal.
An advantage of the alternate embodiment is utilized when a null is present a~ any point in the packet~ Under conventional forward processing condi~ions, after ths occurance of the null, information in the packet is lost~
Usin~ the recovsry process described in the alternate embodiment, infor.l,alion recovery may continue at o~hcr starting points toward ending peints to reeover the majority of data which may have otherwise bcen lost~
The data reoovery processor 107 gen0rally e~lu~ es, detects, tracks tha carrier phase and decodes ~he, infor.llalion in ~he pack~t~ The rccovsr6d data appearing on line 108 from the data recovery proeessor 107 is coupled to a recovered data buffer 113~ The 9 2~73~82 r~cover~d data buffer 113 is a location for storing recovered data 102 from the packet as it is recovered.
After all thc information in a packet is rscovered, the data is coupled from the recovered data buffer 113 t~ a 5 speech deco~er 115. The spe~ch clecoder 115 converts the digitized signal, raceived in the encoded information packet, into speech which may be heard throu~h a sp~ak~r 1 1 7.
NGW referring to FIG. 2, there is shown a block 10 diagram of the da~a recovery proc~ssor ~07. Tha equalizer 203, data detec~or 209, phase detec~or 213 or 215, the loop filter 223 or 22~, the PLL switch 227 and the NCO 231 comprise a phase lock loop (PLL) 232. The detecl~d data from the phase lock loop is cle~odec~ and 1~ stored in the r~cover~d data buffer of FIG. 1. The loop filter, phase dstector and differential decoder each have a forward and r~verse processing mode. The forward processing mode of each funotion is conventionally implemented. The reverse processing mode of sach 20 function is performed by modifing the conventional implemantations.
Data from the receive data buffer 103 is coupled to a correlator 201 and an e~u~li7er 203. Th~ correlator 201 has thrsa purposes. The first purpose is to initializ~ the 25 training sequence for the equali~er 203 by sampling the ma~nitude of the received signal. The second purpose is to initialize the carrier phase of the received signal. The third purpose is to determine the optimum sample rate for clock recovery. The equalizer 203 oorrects time dalay 30 probl~ms caused by time disparsion distortion.
Predetarmined symbols within the packet are stored in vector buffer 205. The location of tha pred~termined symbols within the packe~ is better appreciat~ by rsference to FIG. 6. FIG. 6 repr~sents a chann~l state , 1~ 2~)73182 ~iagram for a TDM packet sequence for a typical land-to-mobile station transmission which the present invention utilizes to its advantage. The TDM system splits tha channel into three packets of information: Rx, Ry and Rz.
5 Each packst is assi~ned to a unique recaivar. The fermat of each si~nal packet is the same. Each signal packet is divided into six adjacant groups of symbols. A signal packet begins with a Sync word 601, havin~ 28 symbols, for synchronizin~ the location of thc packet within the 10 TDM system and for equalizer training. The slow ~CSoci~t~ control channel (SACC) word 603, having 12 symbols, repr~sents commands from the land sta~ion to ths mobile station such as a hand-off required between cells. Next are 130 symbols of data 605 followed by 12 15 symbols representing a digital voice color code ~DVCC) 607. Next are 130 symbols of data 609 followed by 12 symbols reserved (RSV) 611 for future use. The two sets of data symbols, 605 and 609, represcnt a digitize speech signal. The DVCC 607 differentiates behNeen two 20 cells in the TDM system having ths same frequency which eliminates co-channel interference. The DYGC 607 is assigned to a receiver when it entars a new cell.
The sync word 601 in the recoived signal packet, Rx is called the desired sync word sinçe the Rx packet is 25 intended for the r~ceiver in the prefarr0d embodirnent. A
sync word 613 in the adjacsnt Ry packet is called the ~j~cent sync word. A feature of the preferred embodiment of the presen~ invention is to use the desired sync word 601, adjacent sync word 613 and the DVCC 607 30 as starting points for processing information in the Rx packet in both forward and reverse directions in time.
The value and position of these starting points within their respective packets are predetermined and known to the receiver and are stored in vector buffer 205 of FIG. 2.
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2~73182 R0turning now to Fl(;. 2, the veotor buffer 205, storin~ the pradetermined symbois, is coupled to ~he correlator 201 and the equalizer 203. The correlator 201 uses the prsdet0rmined symbols to determine which packet to r~ceiv~. The equalizer 203 uses the predet~l",ined symbols from the vector buffer 2û5 to initializo ths equalizer procsss.
The purpose of the da~a detector 209 is to quantize the information in the packet. A quanti~ad signal at lins 10 211, goner~ted by the data detector 209, is caupled to the fonvard and reverse mode phase detactor 213 and 215, a forward and reverse mode differen~ial decoder 217 and 219 and the equalizer 203. The forward and reversa mode phase cJetaclors 213 and 215 use the equalized signal at 15 line 207 and ~hs quantized signal at line 211 to producs an e~li",a~ of the phase error in the received si~nal. The equaliz~r 203 uses the quantized signal at line 211 to update the coefficients in an algorithm embodied within the e~U~li7er 203. Based on the equalizer's performance, 20 tha equalizer will generate a quality signal at line 111 couplcd to ths mode controller 105 of FIG. 1. Responsive to the quality si~nal at line 111, the mode controlier 105 changes the direction of rscovering the packet.
Th~ phase ~rror es~imat3 si~nal of the forward mode 2~ phase det~ctor 213 is coupled to the forward mode loop filter 223. Likewise, the phase error esli~"ate signal of the reverse mode phase detector 215 is coupled to the reverse mode loop filter 225. The loop filtsrs reduce the distortion in the phase error estimate signals and oontrol 30 the responsa time of the PLL. The filtered outputs of the forward mode 223 and the reverse mode 225 loop filters are coupisd to a PLL switch 2~7. The PLL switch 227, responsive to the control signal at line 109, couples the appropriate filtersd signal from the forward or reverse . . .

12 20731~2 mode loop fil~er 2~3 or 225 to the conventional numeric controlled oscillator (i~lCO) 231. The NCO 231 generates an adjusted phase si~nal which is coupled to the equalizer 203.
The decoder switch 229, responsive to ths control signal 109, couples the appropriate decoded signal from either the forward 217 or reverse ~19 mode differential clecocler to the recovered da~a buffer 113 in FIG. 1.
A block diagram of the e~iu~ er 203 is represented in FIG. 7. The structure of the equ~ er 203 is conventional. A feature of the preferred embodiment is in the utilization of an adaptive algorithm processor 701.
The adaptive al~orithm processnr 701 uses information from the mode controllsr 105 to determine tha dir~ction of signal recovery. Other information from the data detector 209 is used to modify tha gain stages 703 through 706 and to generate the quali~y signal at line 111 for the mode controller 105.
FIG. 3A is a block diagram of th~ conventional phasa detector 213 opsrating in a forward processing mod~. The purpose of the phase detector 213 is to produce an esli~na~e of the phase error si~nal 301. Tha inputs to the phase det~ctor 213 are the er~u~ ed signal at line 207 and the quantized signal at line 21.1. The quadrature-phase component (Q) of the equalized signal at line 207 is combined with the in-phase component (I) of the quantized signal at lins 211 in mixer 303 to produca a positiva error signal 305. Likewise, the quadrature-phase component (Q) of $he quantized signal at line 211 is combined with the in-phase (I) component of the equalized signai at line 207 in mixer 307 to produce a negative error si~nal 309. Tha positive error signal 305 and negative error signal 309 are combined in a summer 311 to produce a phase error estimate signal 301.

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13 2073~82 Flià. 3B is a block diagram o~ a phase detector operatin~ in a reverse processing rnode. The reverse mode phase ~le~ec~or generates a negative phase errsr estimate signal 315 having opposite polarity to the phass error esli",ate signai generated by the forward mode phase datector. This is accomplished by combining a negative unity gain signal, ~:3, wi~h the phase error estimate signal at line 314 in mixer 31~.
FIG. 4A is a block dia~ram of the conventional loop 10 filter 223 operating in a forward proces~ing mode. The loop filter 223 generates a ~iltered phase error signal for the NCO to track the instantaneous phase of the receivad signal packet. The loop ~ilter 223 also tracks the long term frequency variation of the received signal. The loop 15 filter 223 comprises a sac~nd order infinite impulse response filter. The phase error estimate signal at line 301 is coupled to amplifiers 401 and 403. The gain of each amplifier ~01 and 403, determines the response time of the loop filter 223. The output of amplifier 401 is 20 coupled to summ~r 405. Likewise, the output of amplifier 403 is coupled to summer 407. The output of summer 407 is determined from the su~ alion of the second gain signal 404 and a symbol delay si~nal at line 409. Th~
output of summer 407 is coupled to summer 405 and 25 symb~l dclay processor 411. Th~ output of summer 405 is coupled to the numeric controlled oscillator 231 of FIG. 2.
FIG. 4B is a block diagram of a loop filter operating in a reverse processin~ mode. This block diagram is identical to the block diagram of the forward processing 30 mode in FIQ. 4A except the output of the ~ilter 415 is neg~tRd. In FIG. 48 a negative unity gain signal ~13 is combined in a mixer 416 with the output signal of summer 418 at line 419 to genarate the filtered output signal 415. The negative unity gain signal 413 reverses the .

diroction of rot~tion of thc filtered signal at line 419 before it is coupled to the NCO 231. For the reverse mode, the instantaneos phase error of the received signal must be tracked in the opposite direction from the forward moda. The long term frequency variation must also be tracked in the opposite direction ~rom the fonHard mode.
FIG. 5A is a block diagram of a differential decoder 217 oper~iing in a fo~vard processing mode. The quantized signal 211 is coupled to a mixer 501 and symbol delay processor 503. The symbol delay processor 503 is coupled to the mixer 501 throu~h a oonjllgator 50~. The mixer 501 combines ths quantized signal at line 211 and tha delaysd and conjug~ed quantized signal at line ~07 to produce a rotated signal at line 509. The rotated signal at line 509 is coupled to a symbol to binary convsrter 511.
The symbol to binary converter 511 convarts the rotated si~nal cletected in the information packet into a tw~ bit binary pair. The two bit binary pair is coupled to the recovered data buffer 113 of FIG. 1.
FIG. 5B is a block diagram ~f a differential decoder operating in a reverse processing mode. The differ~nce between the reverse mode in FIG. 5B and th~ fon~ ard mode in FIG. 5A is the position of the conjugator 505 as shown in FIG. 5E~. The conjugator 513 is positioned bet~,ve~n the 26 incoming quantizad signal at lina 211 and ~he mixer 515.
Moving the posilion of the conjugator 513 allows the quantized signal to be properly deeo-~ed in the reverse order from the forward processing mode.
Thus, a TDM rec~iver for detacting information in a time-varying signal havin~ a iong duration has been ~isolose:l. The in~orrnation in ~he packet is processed from predetermined starting points, in a direction rssponsive to the intensi~y of the signal over gha dura~ion of the packet.
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Claims (10)

Claims

1. An apparatus for recovering at least one signal packet included in a serial data signal, wherein the packet is formed of multiple sequential symbols and at least one predetermined symbol, the at least one predetermined symbol having a predetermined position and value, said apparatus comprising:
means for receiving the serial data signal;

means for determining a direction of recovery for at least one symbol sequential to the at least one predetermined symbol;

means for recovering said at least one symbol in said determined direction;

means for changing between said determined direction and a second direction; and means for storing said at least one recovered symbol.

2. An apparatus in accordance with claim 1 wherein said means for determining a direction of recovery further comprises means for measuring a time-varying parameter related to said at least one symbol.

3. An apparatus in accordance with claim 1 wherein said means for recovering said at least one symbol in a forward direction further comprises:

means for generating an equalized signal responsive to said serial data signal;
means for generating a quantized signal responsive to said equalized signal;
means for generating a forward direction phase error signal responsive to said equalized and quantized signals;
means for generating a forward direction filtered signal responsive to said forward direction phase error signal;
means for adjusting a phase signal responsive to said forward direction filtered signal;
means for adjusting the phase of said serial data signal responsive to said adjusted phase signal; and means for decoding said quantized signal to generate a forward direction recovered signal.

4. An apparatus in accordance with claim 1 wherein said means for recovering said at least one symbol in a reverse direction further comprises:
means for generating an equalized signal responsive to said serial data signal;
means for generating a quantized signal responsive to said equalized signal;
means for generating a reverse direction phase error signal responsive to said equalized and quantized signals;
means for generating a reverse direction filtered signal responsive to said reverse direction phase error signal;
means for adjusting a phase signal responsive to said reverse direction filtered signal;
means for adjusting the phase of said serial data signal responsive to said adjusted phase signal; and means for decoding said quantized signal to generate a reverse direction recovered signal.

5. A method for recovering at least one signal packet included in a serial data signal, wherein the packet is formed of multiple sequential symbols and at least one predetermined symbol, the at least one predetermined symbol having a predetermined position and value, said method comprising:
receiving the serial data signal;

determining a direction of recovery for at least one symbol sequential to the at least one predetermined symbol;

recovering said at least one symbol in said determined direction;

changing between said determined direction and a second direction; and storing said at least one recovered symbol.

6. A method in accordance with claim 5 wherein said step of determining a direction of recovery further comprises measuring a time-varying parameter related to said at least one symbol.

7. A method in accordance with claim 5 wherein said step of recovering said at least one symbol in a forward direction further comprises the steps of:
generating an equalized signal responsive to said serial data signal;

generating a quantized signal responsive to said equalized signal;
generating a forward direction phase error signal responsive to said equalized and quantized signals;
generating a forward direction filtered signal responsive to said forward direction phase error signal;
adjusting a phase signal responsive to said forward direction filtered signal;
adjusting the phase of said serial data signal responsive to said adjusted phase signal; and decoding said quantized signal to generate a forward direction recovered signal.

8. A method in accordance with claim 5 wherein said step of recovering said at least one symbol in a reverse direction further comprises the steps of:
generating an equalized signal responsive to said serial data signal:
generating a quantized signal responsive to said equalized signal;
generating a reverse direction phase error signal responsive to said equalized and quantized signals;
generating a reverse direction filtered signal responsive to said reverse direction phase error signal;
adjusting a phase signal responsive to said reverse direction filtered signal;
adjusting the phase of said serial data signal responsive to said adjusted phase signal; and decoding said quantized signal to generate a reverse direction recovered signal.

9. An apparatus for recovering at least one signal packet included in a serial data signal, wherein the packet is formed of multiple sequential symbols and at least a first and second predetermined symbol, said at least first and second predetermined symbols each having a predetermined position and value, said apparatus comprising:
means for receiving the serial data signal;

means for recovering at least one symbol sequential to the first predetermined symbol in a forward direction;

means for recovering at least one symbol sequential to the second predetermined symbol in a reverse direction; and means for storing said recovered at least one symbol.

10. A method for recovering at least one signal packet included in a serial data signal, wherein the packet is formed of multiple sequential symbols and at least a first and second predetermined symbol, said at least first and second predetermined symbols each having a predetermined position and value, said method comprising the steps of:
receiving the serial data signal;

recovering at least one symbol sequential to the first predetermined symbol in a forward direction;

recovering at least one symbol sequential to the second predetermined symbol in a reverse direction; and storing said recovered at least one symbol.